The Mona Lisa’s half-smile is famously ambigious. Is she grinning or grimacing? Art historians argue that her mercurial mouth is little more than Renaissance special f/x.

Neuroscientists offer newer theories: It’s about how your brain processes light – or how your eye perceives detail. Maybe all three explanations are right. Leonardo, as good a scientist as he was an artist, probably would have thought so.

From later in the article

…Mona’s enigmatic expression may literally be in the eye of the beholder. “Look directly at her mouth – her smile disappears,” says Margaret Livingstone, a Harvard neurobiologist. “Central vision does not perceive shadowy components well. Look at the eyes, seeing the mouth only with your peripheral vision, and her glorious grin becomes obvious.” That’s because the human eye’s foveal, or central, vision is set up to best perceive detail (and is connected to a disproportionately larger chunk of the brain’s visual cortex); peripheral vision is optimized for broad visual strokes.

To illustrate the effect, Livingstone altered Mona in Photoshop. She used a Gaussian blur filter to emphasize the coarse [B] and medium [C] grains, to mimic how you’d see the painting out of the corner of your eye. Then she applied a high-pass filter to underscore the finer detail [D], isolating the dead-ahead view. The result: As detail increased, Mona’s grin goes flat.

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